1. Field of the Invention
The present invention relates to a back-flow prevention apparatus and to a method of operating an injection apparatus.
2. Description of the Related Art
Conventionally, an injection molding machine has an injection unit. The injection unit has a heating cylinder in which a screw is disposed to be rotatable and to be movable in an advancement/retraction direction. Drive means rotates and advances or retreats the screw. In a metering step, the screw is rotated in one direction or in the forward direction, thereby melting resin supplied from a hopper and accumulating resin melt in a space located forward of the screw head. In an injection step, the screw is advanced so as to inject the resin melt from the space located forward of the screw head via an injection nozzle.
FIG. 1 shows a sectional view of a main portion of a conventional injection unit.
In FIG. 1, reference numeral 11 denotes a heating cylinder. The heating cylinder 11 has an injection nozzle 13 at its front end (left-hand end in FIG. 1). In the heating cylinder 11, a screw 12 is disposed to be rotatable and to be movable in an advancement/retraction direction (left/right direction in FIG. 1). Unillustrated drive means rotates and advances or retreats the screw 12. Notably, an injection cylinder, a motor, or a like device serves as the drive means.
The screw 12 extends rearward (to the right in FIG. 1) within the heating cylinder 11. The screw 12 is connected at its rear end to the drive means and has a screw head 14 at its front end. A spiral flight 15 is formed on the surface of a metering portion 18 of the screw 12 to thereby form a groove 16 along the flight 15.
An unillustrated hopper is disposed at a predetermined position located at a rear portion of the heating cylinder 11. Resin pellets are charged into the hopper.
In the thus-configured injection unit, when the screw 12 is rotated in a forward direction through drive of the drive means in a metering step, resin pellets contained in the hopper drop into the heating cylinder 11 and are advanced (to the left in FIG. 1) through the groove 16. At the same time, the screw 12 is retreated (is moved to the right in FIG. 1).
An unillustrated heater is disposed around the outer circumference of the heating cylinder 11. The heater heats the heating cylinder 11 so as to melt resin pellets contained in the groove 16. Accordingly, when the screw 12 is retreated by a predetermined amount, a predetermined amount of resin melt for a single shot is accumulated in a space located forward of the screw head 14.
Upon completion of the metering step, suck-back is performed; i.e., the screw 12 is slightly retreated without being rotated, to thereby prevent the resin from oozing from the front end of the injection nozzle 13.
Next, in an injection step, the drive means is activated so as to advance the screw 12. The resin accumulated in the space located forward of the screw head 14 is injected from the injection nozzle 13 and charged into the cavity of an unillustrated mold apparatus, thereby filling the cavity with the resin.
In order to prevent backflow of the resin accumulated in the space located forward of the screw head 14 in the injection step, a back-flow prevention apparatus is disposed.
Specifically, the screw head 14 has a conical head body portion 21 formed at its front section (at the left portion in FIG. 1) and a small-diameter portion 19 formed at its rear section (at the right portion in FIG. 1). An annular back-flow prevention ring 20 is disposed around the circumference of the small-diameter portion 19, thereby defining a resin passageway 24 between the small-diameter portion 19 and the back-flow prevention ring 20. A seal ring 22 is disposed at the front end of the metering portion 13 such that the seal ring 22 can contact or separate from the rear end of the back-flow prevention ring 20. Moreover, a cut 25 extending in the axial direction is formed at a plurality of locations on the circumference of the head body portion 21.
Accordingly, when the screw 12 is advanced in the injection step, the resin accumulated in the space located forward of the screw head 14 is urged to move rearward. However, resin pressure causes the back-flow prevention ring 20 to move rearward with respect to the screw 12. Thus, the rear end of the back-flow prevention ring 20 abuts the seal ring 22, so that the communication between the spaces located on the front and rear sides of the seal ring 22 is broken to effect sealing. As a result, the resin accumulated in the space located forward of the screw head 14 is prevented from flowing rearward.
In contrast, when the screw 12 is rotated in the forward direction in the metering step, the screw 12 is retreated. However, due to pressure of the resin, the back-flow prevention ring 20 moves forward with respect to the screw 12. Thus, the front end of the back-flow prevention ring 20 abuts the rear end of the head body portion 21, so that the communication between the spaces located on the front and rear sides of the seal ring 22 is established. At this time, resin moves forward, while passing through the cuts 25, so that resin flow is not hindered.
However, in the injection apparatus, the amount of resin accumulated in the space located forward of the screw head 14 changes during the suck-back operation, because resin at the metering portion 18 moves to the space located forward of the screw head 14 upon performance of the suck-back operation.
Further, since sealing is effected through an operation of advancing the screw 12 to thereby move the back-flow prevention ring 20 rearward, the timing of completion of the sealing operation varies depending on the state of kneading and dispersion of resin, resin viscosity, resin temperature, and the rate at which the screw speed is increased at the time of starting the injection step. Thus, the amount of resin that flows back varies.
Accordingly, even when the injection stroke of the screw 12 for each shot is accurately controlled, the amount of injected resin varies. Further, when the amount of resin that moves during the suck-back operation is excessively small or the timing of completion of the sealing operation is delayed excessively, molded products suffer short shot or like defects. When the amount of resin that moves during the suck-back operation is excessively large or the timing of completion of the sealing operation is advanced excessively, molded products suffer burrs or like defects.
In order to solve the above-described problem, there has been proposed a back-flow prevention apparatus which includes first and second rings disposed on the rear side of the screw head. When the screw is rotated forward, first and second resin passages formed in the first and second rings, respectively, communicate with each other, and when the screw is rotated in the opposite direction; i.e., rotated in reverse, the communication between the first and second resin passages is broken to thereby effect sealing.
In this case, effecting sealing before performance of the suck-back operation prevents resin from moving from the metering portion to the space located forward of the screw head. Therefore, the amount of resin accumulated in the space located forward of the screw head can be stabilized.
Further, since sealing can be effected without advancement of the screw, the timing of effecting the sealing can be stabilized.
However, the conventional back-flow prevention apparatus has a problem such that in some cases the communication between the first and second resin passages cannot be broken through reverse rotation of the screw, resulting in failure to effect sealing.
In such a case, resin accumulated in the space located forward of the screw head flows back during the injection step, with the result that a proper amount of resin necessary for providing a cushion effect cannot be secured. Accordingly, the amount of resin charged into the cavity of a mold apparatus becomes insufficient, so that molded products suffer defects.
An object of the present invention is to solve the abovementioned problems in the conventional back-flow prevention apparatus and to provide a back-flow prevention apparatus and a method of operating an injection apparatus, which apparatus and method can prevent resin from moving due to suck-back and can effect reliable sealing.
To achieve the above object, a back-flow prevention apparatus of the present invention comprises: a screw body; a screw head attached to the screw body; a first resin passage formed on a first ring disposed between the screw body and the screw head; a second resin passage formed on a second ring disposed between the screw body and the screw head to be located adjacent to and be rotatable relative to the first ring; drive means for rotating a screw selectively in one direction and the opposite direction; and a controller.
Rotation restriction means is disposed between the first and second rings such that communication between the first and second resin passages is established when the screw is rotated in one direction, and the communication between the first and second resin passages is broken when the screw is rotated in the opposite direction, to thereby effect sealing.
The controller includes resin-pressure reduction means for reducing the pressure of resin in the space located forward of the screw head after completion of a metering step but before initiation of sealing.
In this case, when the screw is rotated in one direction in the metering step, communication between the first and second resin passages is established, so that resin moves forward from the metering portion that constitutes the screw body via the first and second resin passages and is accumulated in the space located forward of the screw head. Upon completion of the metering step, the pressure of resin in the space located forward of the screw head is reduced by the action of the resin-pressure reduction means.
Subsequently, the screw is rotated in the opposite direction to thereby effect sealing. At this time, the pressure of resin in the space located forward of the screw head has been decreased. Therefore, resin located forward of the screw head is prevented from flowing in the reverse rotation direction with rotation of the screw.
Accordingly, the second ring is not rotated simultaneously with the first ring, so that the communication between the first and second resin passages is broken reliably in order to effect reliable sealing.
Further,: a proper amount of resin necessary for providing a cushion effect can be secured, so that a sufficient amount of resin can be injected. Therefore, the amount of resin charged into the cavity of a mold apparatus never becomes insufficient, so that molded products do not suffer defects such as short shot.
Another back-flow prevention apparatus of the present invention comprises: a screw body; a screw head attached to the screw body; a first resin passage formed on a ring disposed between the screw body and the screw head; a second resin passage formed on the screw head; drive means for rotating a screw selectively in one direction and the opposite direction; and a controller.
Rotation restriction means is disposed between the ring and the screw head such that communication between the first and second resin passages is established when the screw is rotated in one direction, and the communication between the first and second resin passages is broken when the screw is rotated in the opposite direction, to thereby effect sealing.
The controller includes resin-pressure reduction means for reducing the pressure of resin in the space located forward of the screw head after completion of a metering step but before initiation of sealing.
In this case, since disposition of only a single ring is required, the cost of the injection apparatus can be reduced.
In still another back-flow prevention apparatus of the present invention, the drive means rotates the screw in one direction in the metering step, rotates the screw in the opposite direction after completion of the metering step, and holds the screw at a metering-step completion position during the period in which the screw is rotated in the opposite direction.
In this case, since suck-back operation is always effected and an injection step is initiated at the metering-step completion position, variations in the amount of resin injected for each shot can be suppressed.